Experience and testing have shown that softening of the hair fiber prior to shaving is of paramount importance to achieving a comfortable shave. Normal applications of water and shaving preparations take three or more minutes to fully hydrate and soften the hair, thereby providing the least resistance to cutting and giving the most comfortable shave. However, the majority of shavers do not choose to wait that length of time and, on average, prepare their hair for one minute or less prior to shaving, leaving considerable room for improvements in comfort.
The major barrier to penetration of water and other materials into the hair is the cuticle, the hair's outermost layers of highly crosslinked keratin protein. It is known that reducing agents which break the sulfur to sulfur or disulfide bonds, that give hair its strength and rigidity, will modify this cuticle layer and increase the rate of water penetration into the hair after treatment. This can be shown through changes in tensile properties of hair, such as stress relaxation, yield stress and rate of water uptake in treated compared to non-treated hairs. It is believed that this breakage of disulfide bonds makes the hair more porous, allowing water to enter more easily and rapidly.
It is now well-known to utilize reducing agents for waving or straightening hair, and as depilating agents for removal of hair without shaving when used at higher concentrations and pH's. See, for example, Balsam and Sagarin, Cosmetics Science and Technology, vol. 2, pages 39-72 and 167-278 (2nd ed. 1972). The use of a reducing agent in a shaving cream or pre-shave lotion has been suggested in U.S. Pat. No. 3,728,356, U.S. Pat. No. 4,775,530 and U.S. Pat. No.4,935,231 and the possibility of incorporating a depilatory agent in a cartridge razor lubricating strip has been mentioned in U.S. Pat. No. 4,170,821. Compositions containing a depilatory agent and a sodium soap to increase the speed of depilation have been disclosed in U.S. Pat. No. 4,121,904. It is also disclosed in U.S. Pat. No.4,913,900 and EP 551,135 that moisturizing agents and humectants may be added to hair waving compositions containing reducing agents to reduce damage to the hair and improve softness.